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The Constellations


There are 88 constellations.

Ptolemy listed 48 constellations in his Almagest.

In 1590, Tycho Brahe added Coma Berenices (Berenice's Hair), although this seems to have an earlier origin originally.

In 1679, the Southern Cross was separated out from Centaurus.

In 1690, several were added by Hevelius (e.g. Canes Venatici (Hunting Dogs) from a part of Ursa Major and often pictured harassing the heels of the Bear).

Fourteen Southern constellations were supplied by Nicolas Louis de Lacaille.

According to the scheme introduced by Johann Bayer in 1603, the brighter stars are officially classified within each constellation by Greek letters, e.g. Regulus is classified as α-Leonis (using the Latin genitive form of the constellation name). In common usage, the brighter stars are usually known by their (mainly arabic) names, e.g. Regulus in the previous example. However, Alpha Centauri in the South, and therefore only comparatively recently known to the 'Northerners' who made all the rules, is known exclusively by its Latin name.

Although you might habe thought that Bayer would place his stars in a definite sequence from brightest to less bright, it appears that apart from labeling the brightest star as α, the rest are (1) grouped into broad brightness classes and then (2) within each class they were labelled sequentially from one end of the constellation to the other. Note that Betelgeuse is α-Orionis despite being less bright than Rigel which is β-Orionis, implying that Betelgeuse has varied in brightness since receiving its classification.

Ursa Major

Also known by its English name as the Great Bear.

Its seven brightest stars form the The Plough (Britain) or Big Dipper (America). In literature, such as Shakespeare, you can read it referred to as 'Charles' Wain' - Charles apparently being Charlemagne and Wain being an old word for wagon.

The Plough

Merak and Dubhe point towards Polaris in Ursa Minor

Megrez and Phad point 'downwards' towards Regulus in Leo and 'upwards' to Deneb in Cygnus .

The tail of the Plough points towards Arcturus in Bootes. Following this line along, you arrive at Spica in Virgo.

The W-shaped constellation of Cassiopeia lies on the other side of Polaris to Ursa Major.

Mizar is actually a double, with Alcor. In this particular case the stars are close together in the sky as seen by us, but are not actually in close contact in the real sense. Mizar itself though is actually a binary and can be viewed as such through a telescope. (It was the first telescopic discovery of a double star, by G.B. Riccioli in the 1600s). Furthermore, each component of this binary is itself binary - they are spectroscopic binaries, the first spectroscopic binaries to be discovered, in 1889. Alcor is also a spectroscopic binary, so in fact we have six stars all told.

The magnitudes of the individual stars are

Alioth 1.8
Dubhe 1.8
Alkaid 1.9
Mizar 2.1
Merak 2.4
Phad 2.4
Megrez 3.3

from which you can see that the straightforward 'greek alphabetical' order is not obeyed precisely.

The Plough

Interesting objects in/around Ursa Major include

  • M51, the Whirpool Galaxy, the first Spiral to be seen as a spiral. A face-on spiral. NGC5195 appears to be interacting with M51.
  • M63
  • M81 Spiral Galaxy.
  • M82 a Starburst galaxy.
  • M97 The Owl Nebula, a planetary nebula.
  • M101 Face-on spiral.
  • M106
  • M108
  • M109



Orion contains two first-magnitude stars, Betelgeuse and Rigel. Betelgeuse is a red Supergiant, a variable star and potential Supernova. The three stars of Mintaka, Alnilam and Alnitak make up Orion's Belt, below which is M42, the Orion Nebula, an HII region.


Following the belt downwards to the left leads to Sirius in Canis Major, the brightest star in the sky.

Above Sirius and to the left of Orion, lie Procyon in Canis Minor and Castor and Pollux in Gemini. In a light polluted sky, these stars will probably be very obvious (paradoxically). Incidentally, Castor is actually a 'six-star' system - it is a double with each component being a spectroscopic binary and there exists a fainter component which is also a spectroscopic binary.

Following the belt upwards to the right leads to the orange/red star Aldabaran in Taurus (and the Hyades open cluster), and further on to the Pleaides, an open cluster. Yet further on, the line of projection will come to Pegasus

Scorpius, the Scorpion who killed Orion, is in the Sky when Orion is not.


Occasionally, and unofficially, referred to as the 'Northern Cross', which is worth mentioning precisely because it does has this shape. It is more distinct than the Southern Cross in that it has a bright star marking its center. The 'long branch' of this cross has at one end :

Deneb , one of the brightest stars in the sky (as seen by us - the 19th. brightest) but whereas most of the bright stars are bright because they are fairly close to us, Deneb is intrinsically very bright and is at a very large distance from us (1630 Light Years).

and at the other end

Albireo a colorful double star - a third magnitude yellow star and a fifth magnitude blue star.

Cygnus lies in the Milky Way. An intervening dark nebula causes the Milky Way to be split in two thru most of Cygnus (Altair Rift?).

Pegasus and Andromeda

Stating from Cassiopeia, a line joining the central star of the 'W' and the star of the 'W' immediately to its right will point towards the Square of Pegasus. This Square of Pegasus is notoriously harder to locate than you might think. You need to take note of its size (and remember that the Sun and Moon subtend half of a degree) and to realise that it will be probably more 'slanted' in the sky than shown on a diagram.

The star in the top left hand corner of the Square, Alpheratz, is now a part of Andromeda - actually α-Andromedae (it did previously belong to Pegasus). Leading off from this star at 10-11 o'clock, there are two other stars which together form the 'backbone' of the Andromeda constellation. Off at 'right angles' from the middle star lies M31, the Andromeda Galaxy, claimed to be a naked eye object.

This line of three stars in Andromeda points to Pegasus.

The star at the left hand end of this 'backbone', Almach (γ-Andromeda) is a yellow/blue binary.

If you know anything about Greek mythology, you will know that these names are all connected. Perseus went to the resue of Andromeda, whose mother was Cassiopeia and father was Cetus (another adjacent constellation). Perseus carried out the rescue with the help of the flying horse, Pegasus.

The two 'right-hand' stars of the Square point downwards to Formalhaut in Piscis Australis, although this is very low from Britain.



Many objects to be seen in the constellation are from the Virgo Cluster

Virgo Cluster

The constellation is in the form of the letter 'Y', with Spica at the base. The 'bowl' section is made more 'complete' by including Denebola (from Leo). The star at the top of the 'stem' (or at the intersection of the 'Y' is Porrima (γ). It is a binary of the type which are sometimes called 'headlights' in that its two components have similar intrinsic luminosities, temperatures and masses (for Porrima actually yellow stars, magnitude 3.6).

Virgo is actually the second largest constellation in the sky by area.

Scorpius and Sagittarius

Called Scorpius in astronomy, not Scorpio

Sagittarius and Scorpius RegionVirgo Cluster


Often called the 'Keystone', which is the central part of a 'comprehensive' constellation depiction and the most obvious poertion of it. M13 lies on the 'right-hand vertical' of the Keystone.


The Circlet, 5 stars



Southern Cross

Southern Cross

Modern Notation

Obviously very few of the constellations actually look like the objects that they are supposed to represent. This was made worse when the Southern constellations were 'constructed' and for the most part purely arbitary names were used with no pretence that the constellation actually looked like the object.

So in many cases, alternative names have come into being. Generalized descriptions are also useful in other cases. The following are examples

Great Bear Plough, or Big Dipper
Little Bear Little Dipper
Cassiopeia W
Virgo Y
Sagittarius Teapot
Taurus V
Aries Three Stars
Canes Venatici Two Stars
Auriga Pentagon
Cepheus Pentagon
Pisces Circlet (of 5 stars)
Delphinus Kite
Corona Borealis Semi-Circle
Pegasus Square
Hercules Keystone
Cygnus Northern Cross
Vega Parallelogram
Corvus Trapezoid/Tilted Kite/Sail

Crab Nebula

Big Red Spot on Jupiter